I've got 4.5 hours on an 290 HP IO 540, w/ James Cowl and Plenum, 1st and 2nd flight showed oil temps peaking @ 232. OAT 75F 4500 ft, 24.5 sq. I already have the airflow 2006X installed. I removed the left side heat muff and scat tubes, cut the bottom of the lower cowl forward 1" yeilding aprox 4.5 sq in increase in lower cowl exit area, and cleaned up some other obstructions in the lower cowl. 3rd & 4th flight showed oil temps peaking @ 215, OAT 70F 3500 ft 25 sq. The oil temp issue has been reported by several -10 builders, I know of 2 that have plumbed a 2nd oil cooler and it seems to have done the trick. I know of one builder that took extensive differential manometer (pressure) readings at various points in the lower cowl. they all consistently indicated that the lower cowl pressures were too high thus impeeding the flow of air through the oil cooler (complicated by Van's obtuse mounting of the O/C). I have spoken with several who have installed louvers, and it appears to me that this has consistently shown good results. I have concluded (?) that for the james cowl/plenum there is not enough exit area in the lower cowl and that is impeeding exit air and increasing the pressure. Several builders in this situation are also reporting CHT issues.
Today I finished installing louvers to the bottom of my cowl.
I'll fly again on Monday and report any changes.